Problem while maintaining the sessions for different folders

I am working on an aplication which contains two different portals (admin and members).http://localhost/app/ is used for the members login and http://localhost/app/admin is used for admin's login.
When I log in into members and admins portal both works fine but if I logout from one portal another portal logged out automatically.

I checked that the session file created in /tmp/ directory stores the sessions information for both the portals in a single file which causes the above problem. The work around I think is to save the session information of both portals in different directories. I searched a lot for this but didn't get any resolution :(

I have tried ini_set('session.save_path',$_SERVER['DOCUMENT_ROOT'].'/admin/sessions') and ini_set('session.save_path',$_SERVER['DOCUMENT_ROOT'].'/sessions') for both admin and users respectively before session_start() but login script gets halt. :(